1. Field of the Invention
The present invention relates to a method for forming fine concavo-convex patterns in which relief holograms and diffraction lattices are recorded, a method for producing an optical diffraction structure composed of fine concavo-convex patterns, and a method for copying an optical diffraction structure.
2. Description of Related Art
Relief holograms and diffraction lattices are for recording interference fringes formed by interfering laser beams in form of fine concavo-convex patterns. The fine concavo-convex patterns are composed of several hundred to thousand fine projections in every 1 mm length.
In the case of copying the fine concavo-convex patterns, a copying forme is produced from a master hologram in which the interference fringes of laser beams are directly recorded and the concavo-convex patterns of the copying forme are transferred to a resin material to copy the fine concavo-convex patterns by mass production.
In a conventional case of forming a copying forme from a master hologram, it has been known to employ a method for producing diffraction patterns of phase diffraction lattices, phase holograms and the like by stamping the patterns in a thermoplastic resin with a casting die (a mother die) by pressure and heat. However, since formation of such plane patterns involves formations of a large number of phase diffraction lattices by repeating stamping many times in the thermoplastic resin and accordingly raised parts are generated in the boundary regions between the parts which are heated and pressurized and parts which are neither heated nor pressurized, it becomes difficult to arrange the patterns without any joining parts. Further, since the casting die is made of a metal, it has a high heat capacity and therefore, there occurs a problem that the previously stamped and neighboring patterns are forcibly eliminated in the peripheral region where stamping is carried out newly.
There is a method for forming fine concavo-convex patterns sharp in the stamping circumferential region without any raised parts to be obstacles by stamping a predetermined region in a simple manner so as to solve the above-mentioned disadvantages (reference to Japanese Patent Application Laid-Open No. 61-20723).
This method for forming a fine concavo-convex pattern is a method for forming concavo-convex patterns by stamping a predetermined surface area of a casting die having fine concavo-convex patterns to a thermoplastic resin and is carried out by heating approximately point-like generatrices of the thermoplastic resin at the focal point of an irradiation source by using the irradiation source and thus stamping the fine concavo-convex patterns of the casting die only to the generatrices so as to form the surface patterns in form of an assembly of a plurality of generatrices.
Particularly, a transparent substrate is fixed on a pressing plate and a material (a plastic layer colored by a colloidal carbon) which is thermoplastic and absorbs light beam is set on the opposed side of the plate and a no heat generation type casting die (made of a nickel alloy) is placed on the opposite to the plastic layer and a pressing force is generated in a dotted region between the casting die and the plastic layer by using a stamp having projected faces only in the focal point region. Further, an irradiation source comprising laser, an optical modulator, and a lens system is set in the opposed side to the side of the casting die in relation to the plate.
When laser beam (radiant ray) is beamed to the plastic layer for forming an image at the focal point, the plastic layer absorbs the radiant ray and is heated at the focal point region. Simultaneously with the heating by beaming radiant ray, pressure for stamping is applied by the stamp, so that point-like generatrix parts in the focal point region of the plastic layer are heated to the thermoplastic temperature and these parts are deformed plastically corresponding to the fine structure of the casting die and even after cooling, the fine structure is fixed and thus predetermined fine concavo-convex patterns are formed.
Also, as a method for forming an optical diffraction structure provided with the three-dimensional impression of a holographic image as well as the brightness and clearness of a diffraction lattice, there is a well-known technique for obtaining one optical diffraction structure in which interference fringes for forming the diffraction lattices and holographic images coexist by carrying out exposure and development of photographic images and diffraction lattices on a dry plate through a mask pattern (Japanese Patent Application Laid-Open No. 59-99475).
Further, production methods involving directly drawing lattice patterns of diffraction lattices and interference fringes of holographic images simulated using a computer by electron beam (Japanese Patent Application Laid-Open Nos. 6-337315 and 11-24539) have been known in these years.